A typical data processing printer operates on a line-at-a-time basis, using forceful impacting of printing elements on multiple paper sheets. The printing elements may be front mounted, as with a dot matrix printer, in which case a platen or bed mechanism of some kind is used to provide a base against which the printing elements may react. The platen is advantageously movable away from the paper, for freedom in loading paper and for adjustment for different paper thicknesses. Such a mechanism is shown in recently issued U.S. Pat. No. 3,941,051 entitled "Printer System" and assigned to the assignee of the present invention. A variety of other printer mechanisms in which different structures are separable to permit paper insertion have also been employed. In general it is considered highly desirable to affirmatively sense both the operative position of the platen or equivalent structure, and the presence of paper in the system. Operation without paper would damage the printing elements and platen, while operation with paper in but the platen withdrawn would result in loss of data. The platen must, however, be open (spaced apart from the paper path) for easy paper loading.
The "paper out" condition is generally sensed in such printers by the actuator arm of a small electromechanical switch, such as the widely used "Microswitch" (a trademark of Honeywell, Inc.). In most prior art structures the actuator arm of the switch is easily depressed and presents no barrier to paper loading. However, once the paper is inserted it cannot be adjusted sideways, and if improperly aligned can only be withdrawn and reinserted. The switch must also be unresponsive to normally encountered variations in the paper, such as are introduced by bulges, fold lines, and perforations in fan fold paper. These variations, however, can exceed in distance the short operating travel of the switch actuator. Thus with existing systems the actuator must be precisely positioned and accurately set, but can still provide erroneous paper out indications because of paper variations. The platen open status normally requires a second sensor, which may be another switch or may be a contact arm spaced to engage the withdrawn platen.